Energy harvesters are known that convert vibrational energy into electrical energy. The electrical energy produced can then be stored or used by other devices. For example, the vibrations of an air conditioning duct can be converted into electrical energy by an energy harvester and the electrical energy then used to power a sensor that measures the air temperature in the duct. The sensor does not require electrical wiring to a remote source of power or periodic battery changes.
There are a variety of such devices for generating electrical power from vibrations, oscillations or other mechanical motions. Generally such devices are categorized as inductive, capacitive, and/or piezoelectric devices. While each of the known types of vibrational energy harvesters have different advantages, they also have drawbacks such as: the need for heavy, powerful permanent magnets (in the case of inductive energy harvesters) to produce a sufficiently large flux density; an auxiliary source of power, such as a battery (in the case of capacitive harvesters); the need for large vibration frequencies and/or a heavy inertial mass to generate sufficient vibrational energy for harvesting (for all types of vibration energy harvesters); undesirable levels of damping and noise generation from interaction between the locally generated magnetic field and nearby metallic parts (for inductive harvesters); and other disadvantages such as size or weight that make them unsuitable for use in a remote or generally inaccessible location.
As shown in FIG. 1, a typical magnetic device 2 to harvest vibrational energy consists of a permanent magnet 3 (an internal local field source) attached to a housing 7 and connected by a spring 4 to a passive magnetic field sensor 5 (e.g., an induction coil or a passive magnetostrictive/electroactive field sensor). External vibrations cause a relative motion of the magnet 3 and sensor 5, producing an electrical voltage across a load 8 and a current through the load. In addition to requiring a local magnetic field source (e.g., a permanent magnet disposed adjacent to the field sensor), these devices also typically include an inertial mass 6 (also known as a proof mass) to increase the vibrational energy generated. The inertial mass may be rigidly attached to the sensor (as shown), may be the same or separate from the permanent magnet, or may comprise part of the sensor itself. As a further alternative, the magnet may comprise part of the moving proof mass, as opposed to being fixed to the housing. In each case, the inertial mass and the permanent magnet increase the size and weight of the device.
It would be desirable to provide a magnetic energy harvester that does not require a local magnetic field source as part of the device itself. It would also be beneficial to provide a magnetic energy harvester that does not require a source of vibrational or other mechanical motion.